1. Field of the Invention
The invention relates to methods and apparatus for estimating the volume of fluid absorbed by a patient during a surgical procedure.
2. Fluid Absorption in Endoscopic Surgery
Endoscopic surgery is frequently performed on the uterus (transvaginally) and the prostate gland (transurethrally). These anatomic approaches generally require that an endoscope be inserted through an orifice into a body cavity and that excised tissue be removed from the surgical site through the same orifice. To maintain the surgeon's clear view of the surgical site and to facilitate continuous removal of blood and small tissue fragments, a substantially continuous flow of relatively low-pressure (frequently electrically non-conducting) irrigation fluid from an external reservoir is maintained through the endoscope. The fluid is frequently formulated as a substantially isosmotic crystalloid solution comprising one or more nonelectrolytes such as glucose, urea, glycine, mannitol, or sorbitol. Pressurization of the fluid often results in mild local distention of tissues, which can lead to absorption of a portion of the fluid through surgical lesions.
It is recognized that incidental absorption of relatively small quantities of irrigation fluid is virtually unavoidable during endoscopic surgery and that for most patients the clinical importance of such small quantities would be negligible. But certain patients (for example, those in heart or renal failure) may be particularly sensitive to the changes in intravascular volume that absorption of even small quantities of irrigation fluid can cause. And absorption of larger quantities may well have adverse clinical consequences even in normal patients.
For example, if irrigation fluid under a pressure head of approximately one meter is applied to the urethra and bladder of an adult patient undergoing transurethral resection of the prostate (TURP), one may estimate that about 10 to 30 ml of irrigation fluid will be absorbed per minute of resection time. On the other hand, the absorption of as much as 6 to 8 liters of irrigation fluid has been documented during TURP's extending over a period of 2 hours. Following absorption of such large amounts of irrigation fluid, dilutional hyponatremia and/or over-hydration may be manifest in seizures, coma and death.
In part, this is because a patient may retain only 20% to 30% of absorbed irrigation fluid within the intravascular space. Free water generally moves to the interstitial space, where it may substantially increase the likelihood of pulmonary and cerebral edema formation. Whether or not a given patient will actually develop pulmonary and cerebral edema, however, depends on several factors, including that patient's cardiovascular status, the amount and rate of onset of the irrigation fluid load, the initial fluid volume status of the patient, and the amount of blood loss during the operation.
Thus, careful monitoring of fluid intake by the patient in real time may be very helpful when combined with other preoperative and interoperative assessments. Note that calculation of the total fluid intake by a patient during an endoscopic surgical procedure includes irrigation fluids (having no electrolytes) and intravenous fluids (which may or may not contain electrolytes). Nonelectrolyte fluids administered intravenously may occasionally represent a clinically significant fraction of the total free water intake. When this occurs, the intravenously administered free water volume should be added to an estimate of irrigation fluid absorbed.
Measurement of fluid infused through an intravenous catheter is relatively easy, although subject to about 10% reading error in collapsible plastic bags. In contrast, estimation of irrigation fluid absorption is error-prone because irrigation fluid may drain continuously from both the endoscope and the body orifice in which it is inserted (i.e., the vagina or urethra as the case may be). Draining fluid is commonly distributed over the surgical drapes, the operating table and floor, as well as to containers resting on the floor. Incidental absorption by and adsorption to various operating-room surfaces, as well as losses in handling due to spillage and splashing, make irrigation fluid recovery uncertain. This reduces the clinical usefulness of fluid absorption estimates.
Nevertheless, because the complications of excessive intravascular volume expansion and/or serious free water overload can be lethal to the patient, the surgeon must have sufficient warning of trouble to take corrective action. Such warning may be provided by having operating room personnel monitor the amount of non-electrolyte intravenous fluid and irrigating fluid drawn from source bags on an IV stand and subtract the amount of irrigation fluid collected from the endoscope drain and surgical drapes. As the procedure progresses, comparisons of the volume remaining in the source bags relative to the volume collected provides an estimate of absorbed fluid volume. Because some of the irrigation fluid is inevitably lost, the estimate of absorbed fluid will tend to be high, thus providing an earlier warning than if actual measurements of absorbed fluid were available. This safety margin may be reduced or eliminated, however, if reading errors are taken into account. For example, the tolerance on fluid volume measurements in the supply bags is about .+-.10%, so for each three liter source bag, the error band may be about 600 ml wide.
A more convenient and more accurate method that has been proposed requires hanging the source bags and a collector bucket on the same spring scale and by that means directly and automatically comparing a decrease in source fluid weight with an increase in collected fluid weight. This suspended system, while simple, is inconvenient because it is fixed to a support in the ceiling. Additionally, such a suspended system may has a tendency to rotate around the suspension member, kinking source and/or drainage tubes and complicating weight measurements because of varying lateral and vertical load components. Accurate weights can only be obtained if the suspended system is actually vertical, but in general it will not be because of the lateral forces applied by the various fluid lines attached to it. Finally, errors inherent in accounting for fluid source bags added to the scale and fluid drainage removed from the collector make use of a suspended system in elective surgery problematical.